In semiconductor technology isotropic etching is non-directional removal of material from a substrate via a chemical process using an etchant substance. The etchant may be a corrosive liquid or a chemically active ionized gas, known as a plasma (Wikipedia—http://en.wikipedia.org/wiki/Isotropic_etching). By comparison anisotropic (or non-isotropic) etching is direction-sensitive, as material is removed from the substrate according to the crystal orientation of the silicon substrate.
Isotropic-anisotropic etch processes have been used to improve metal step coverage for both contact and via etch processes. Initially, the isotropic etch process was performed using a “wet” or buffered oxide etch (BOE) followed by a “dry” or plasma oxide etch for the anisotropic role. As geometries decreased, wet etches yielded to “dry” isotropic etches. The ability to control the amount of undercut for the isotropic etches is predicated on the gas chemistry, RF power, etch time, temperature, load size and material being etched. The “dry” isotropic etch immediately indicates that this is a “chemical” etch. The etch rate of a chemical etch is controlled by the temperature of the solution or in this case, the temperature of the wafer.
Isotropic etch repeatability has consistently been a problem with dry etches. Even when the closest manufacturing specifications are implemented, parameters such as chamber pressure (i.e. the speed at which the throttle valve operates) and gas flows, will differ from one etch system to the next. This is a leading cause in the variation of isotropic etch depths.
A common approach to reducing such process variability is to process single wafers in each chamber. However, chamber temperature increases result in greater “over etch” or greater isotropy between the first and last wafers in a lot.
Another approach to reducing isotropic etch process variability is to individually adjust the etch times for each etch recipe while using scanning electron microscope (SEM) photos to confirm both etch depth and critical dimension control. However, process variability due to pumping speed, mass flow controller stability, chamber pressure stabilization, RF tuning and other factors yield different etch results on different systems.
Accordingly, what is needed is a method of forming a semiconductor device that overcomes the above-described operational issues. The method should be cost-effective, easily implemented, efficient, and have good performance characteristics. The present invention addresses such a need.